Temperature-sensitive mutants of Bacillus subtilis defective in competence for DNA transformation have been isolated. Higher frequencies of transformation defective mutants were obtained by mutagenizing strain GSY1027, defective in repair of ultraviolet radiation damage but not recombination. Many of the mutants were not classical recombination-defective as indicated by their resistance to mitomycin C and methyl methane sulfonate. Studies are in progress to map the mutational sites by bacteriophage PBS1 transductions. The mutants will be characterized as to the stage of transformation blocked at the non-permissive temperature. The major endodeoxyribonuclease found in competent strains of B. subtilis has been isolated from a mutant producing elevated levels of the enzyme. It has been purified to apparent homogeneity and a molecular weight of 35,000 daltons determined. Studies of the possible role of this enzyme in the processing of transforming DNA following uptake by competent cells will be made. BIBLIOGRAPHIC REFERENCES: Burke, W.F., and Spizizen, J. 1976. Purification and properties of the heat-activated endonuclease in Bacillus subtilis. Abstracts of the Annual Meeting of the American Society for Microbiology, p. 157. Burke, W.F., and Spizizen, J. 1976. Characterization of the heat-activated endonuclease in Bacillus subtilis. Fed. Proc. 35: 1619.